Outbreak of Legionnaires’ disease at the American Legion convention in Philadelphia in 1976

Table of contents:

1. Introduction

2. The first Legionnaires' disease outbreak in the USA.

3. Challenges and lessons learned since LD epidemic of 1976.

4. Current advances in the diagnosis of LD

5. Current methods for public prevention
5.1 Prevention of LD hospital outbreak
5.2. Prevention of LD epidemic in the community
5.3. Prevention of travel associated LD outbreak.

6. Conclusion

7. References.

1. Introduction

Legionnaires disease (LD) is a bacterial pneumonia dated back to the mid of 20th century. Its name came from the fact that it was first described within the legionnaires in Philadelphia, United States of America (USA) in the 1970s(1,2). It is a debilitating infection and causes many complications which, if not taken care of, are lethal. The causative bacteria are called Legionella pneumophila; they are also associated with a non-pneumonic form called Pontiac disease.

LD is thought to be transmitted through a mist of aerosols from contaminated water sources like respiratory therapy equipment, showers, decorative fountains, cooling systems, potting soil, humidifiers, and ice machines(3). The bacteria replicate in the water, and the vulnerable individual gets infected by inhaling the small water droplets called aerosols which get into the body through respiratory tract. The vulnerable individuals have been identified to be those with weakened immune system like smokers, alcohol abusers, cancer patients, patients with final stage renal disease and diabetes mellitus, advanced age, people living with human immunodeficiency virus (HIV) or acquired immunodeficiency syndrome (AIDS) and those receiving drugs which suppress the immune system like steroids(3).

Epidemiologically, LD occurs worldwide mostly in the form of outbreaks and its diagnosis depends on the availability and capacity of the laboratories to perform a correct diagnosis. It is well diagnosed in developed countries due to the availability of resources. Bacterial culture is the gold standard, but there are also rapid serologic diagnostic tests which will be mentioned later in this easy.

The incidence varies worldwide, and this is due to the strength of surveillance and reporting systems. In the state of New York, the incidence trend has been reported to be increasing every year as seen in figure 1, and death rates are as high as 13% in treated patients(4). World Health Organization, WHO (5) reports that in the USA, Europe, and Australia there are approximately 10–15 cases detected per million.

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Figure 1: Annual number and incidence (no. cases/100,000 population) of Legionnaires’ disease cases, New York, New York, USA, 2002–2011. Source: Farnham A, et al. Legionnaires’ disease Incidence and Risk Factors, New York, USA, 2002–2011. Emerg Infect Dis, 2014; 20(11):1795-1802.

Clinically, LD has typically an incubation period of 2 to 10 days (1, 3, 4, 6) and modeling of LD outbreaks support an incubation period of six to seven days (7). The early symptoms include fever, loss of appetite, headache, malaise, and lethargy. A few patients may report muscle pain and confusion. There is usually a mild non-productive cough at the beginning; some patients develop phlegm. Progress to severe pneumonia and multiple organ failure occurs if no treatment is administered.

In this essay, description of the first reported outbreak of LD in the USA will be given followed by challenges and lessons learned until recently. Then, briefly, there will be a mention of current LD diagnostics tools and current methods used for public prevention.

2. The first Legionnaires' disease outbreak in the USA.

The first significant outbreak of LD occurred in summer of 1976 in Philadelphia, the USA among people who attended the 58th annual convention of the American Legion of the state of Pennsylvania(1). Official activities took place in the Bellevue-Stratford Hotel, and there was also another convention of the American Legion Auxiliary conducted in another hotel at the same time. Prior to1976, there were several pneumonia outbreaks resistant to penicillin in which the causative agent was not known with similar features, but currently, they are believed to have been caused by Legionella pneumophila ( 8). This microorganism was isolated and identified from lung tissues of dead patients to be a thin-walled gram-negative bacilli bacterium (2).

In the outbreak investigation, both clinical and epidemiological criteria were considered. Clinically a patient was supposed to have a fever, 38.5oC or higher and cough or any fever with chest x-ray abnormality with onset between 1st of July and 18th of August. Epidemiologically the case was supposed to have been in the convention from 21st to 24th of July 1976 or to have entered the hotel. Those cases with only clinical criteria were termed to have "broad street pneumonia."

Active case findings consisted of the search for patients in hospitals using public health trained nurses, and passively, a telephone hotline was set up so that the public could report any suspected case. Various surveys were conducted including hotel-guest surveys in four hotels to assess the rate at which the cases were meeting the clinical criteria, hotel-employee surveys to determine if they were affected by the disease, roommate surveys, hospital surveys in three hospitals to determine if the cases meeting the clinical criteria were occurring in other places apart from the convention and the Bellevue-Stratford Hotel, surveys of reported deaths due to pneumonia and influenza among Philadelphia residents from June to September of 1974, 1975 and 1976 and legionnaires survey questionnaires. Two more case-control surveys were conducted. Samples were collected from the hotel environment, workers, and patients.

The entire outbreak included 182 patients who met clinical and epidemiological criteria. Of those, 82% attended the convention, and about 81% were hospitalized with a case fatality rate of 16% (1). The clinical features were similar to those described in the introduction. Half of those who died presented shock, and a majority kidney malfunction. From the record of 94 hospitalized patients, more than half presented leucocytosis, i.e., increased white blood cells. Chest radiographs were abnormal in 90% of hospitalized cases. The attack rate was found to increase with age, similar to what has been found in other outbreaks (4). Case fatality among those who met criteria for broad street pneumonia was 13%. The epidemic curve (Figure 2) showed a rapid increase soon after the start of the convention, then a plateau followed by a rapid decline of cases

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Figure 2: Epidemic curve of LD outbreak in 1976, Philadelphia. Source: Fraser, D.W. et al. Legionnaires’ disease. Description of an epidemic of pneumonia. N Engl J Med. 1977; 297:1189-97.

The incubation period ranged from 2 to 10 days the except for two patients. The analysis showed that the Bellevue-Stratford Hotel was the primary site of exposure. It is also arguable that the causative agent was in the hotel for a long time because serologic tests found a higher proportion of titers in those who started to work before 1975. No association was found between LD and food or ice drinks served, but there was a significant association with drinking water at the hotel in those who met case definitions. Airborne transmission could not be proven but it was suspected as in other similar outbreak investigations (9,10).

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